In each of the fields of materials to be used for household electric appliances, automobiles and architectures, there has widely been used a surface-treated metal material, which has been obtained by subjecting the surface of a metal material, such as zinc-based plated steel sheet and aluminum-based plated steel sheet, to a chemical conversion coating, such as phosphate (or phosphoric acid salt) treatment, chromate treatment and chromate-free treatment, for the purpose of imparting corrosion resistance thereof, or adherence thereof with a top coating film. Particularly, in recent years, with a worldwide rise in the environmental consciousness, the ratio of the application of a chromate-free coating film, which does not use an environmental load substance, such as hexavalent chromium, is increasing.
In a case where a surface-treated metal material is used as a material for household electric appliances, automobiles and the like, a finished product is obtained by subjecting the surface-treated metal material to through various processing steps or transport steps. For example, a handling scratch in the processing step or an abrasion scratch in the transport step gives rise to a quality problem in many cases, and therefore, a surface-treated metal material having excellent scratch resistance is strongly demanded. Out of the processing steps, a most important step is a press forming (or press molding) step. At the time of the press forming, a press mold and a chemical conversion coating film come into contact with each other under a high surface pressure, to thereby cause the rubbing therebetween. Accordingly, if the chemical conversion coating film lacks strength, there may be caused damaging or separation of the coating film, to thereby incur the deterioration in the appearance quality thereof, such as blackening, or the reduction in the corrosion resistance thereof. On the other hand, the press forming also involves the deformation of the material in itself, such as elongation or compression thereof. Accordingly, if the chemical conversion coating film lacks flexibility (or elongation), the coating film cannot follow the deformation of the material, and the cracking or separation thereof may be produced in the coating film to thereby incur the deterioration in the quality of appearance, such as whitening, or the reduction in the corrosion resistance. That is, in order to guarantee a good press formability or corrosion resistance after the shape processing, it is important to apply a chemical conversion coating film excellent in both the strength and the flexibility (or elongation). Particularly, in recent years, a surface-treated metal material having excellent press formability and corrosion resistance after the shape processing thereof, which is less likely to be deteriorated in the appearance or corrosion resistance, even when it is subjected to a severe press forming process, for example, a heavy ironing-(ultra) deep drawing process for the application such as motor cover and cartridge-type tank, or a non-oiling process for the purpose of omitting an oiling step or a degreasing step.
Various types of chromate-free coating films have been heretofore devised. In comparison with a coating film comprising an inorganic compound as a main component, the coating film mainly comprising an organic resin is characterized in that it has an excellent effect of blocking a corrosion factor, as well as excellent lubricity (i.e., an effect of reducing the frictional force against a press mold), or has an excellent flexibility, and accordingly, it is advantageous in view of corrosion resistance and press formability. In particular, when a coating film can be formed by using an aqueous coating material mainly comprising an aqueous organic resin, the harm thereof to the environment and fire risk can be minimized as compared with those in the case of an organic solvent-type coating material, and such a coating film can meet the growing requirement for environmental consciousness. In addition, the coating equipment to be used therefor can be relatively simple and the above coating film can be produced by using equipment involving relatively low-temperature baking, and this not only enables minimizing the equipment investment cost, but also is advantageous in view of the energy cost involved in the production. Herein, the aqueous organic resin refers to a water-soluble organic resin or a water-dispersible organic resin, which is originally insoluble in water but, as in an emulsion or suspension, can be put into a state of being finely dispersed in water. The aqueous coating material refers to a coating material using a solvent mainly comprising water, and does not fall within the scope of the organic solvent, which is defined in the Industrial Safety and Health Law in Japan.
In the coating film mainly comprising an organic resin, one of the values of physical property affecting the characteristic of a coating is the glass transition temperature of the organic resin. The glass transition temperature is the temperature at which the organic resin changes from a vitreous state to a rubbery state, so as to cause a great change in the characteristic of the organic resin between those before and after the transition temperature, and this is a physical property value markedly affects the scratch resistance or press formability of the coating film. The glass transition temperature is also one of the indices generally indicating the strength or flexibility (or elongation) of an organic resin, and it is known that as the glass transition temperature of an organic resin becomes higher, the strength of the organic resin is increased but the flexibility (or elongation) thereof is reduced.
Heretofore, for the purpose of improving the performance of a coating film, several techniques to be used for a coating film mainly comprising a resin having a specific glass transition temperature have been disclosed. For the purpose of improving the corrosion resistance or the scratch resistance of the coating film at the time of the forming process, for example, Patent Document 1 discloses a technique to be used for a urethane-based resin coating film mainly comprising a polyurethane resin having a glass transition temperature of 100° C. or more; Patent Document 2 discloses a technique to be used for a coating film to be formed from a water-dispersible metal surface-treating agent mainly comprising an ethylene-unsaturated carboxylic acid copolymer having a glass transition temperature of 50 to 70° C.; Patent Document 3 discloses a technique for a coating film to be formed from a surface treating agent for a zinc-based plated steel sheet containing a urethane resin having a glass transition temperature of −40° C. to 0° C. and a water-soluble epoxy resin; and Patent Document 4 discloses a technique for a two-layer coating film comprising a lower coating layer formed from a solution of primer agent containing an aqueous organic resin, and an upper coating layer formed from an aqueous lubricating coating material containing an aqueous organic resin having a glass transition temperature of 70 to 200° C.
In the case of the coating films disclosed in Patent Documents 1 and 2, where the main component is one kind of an organic resin having a specific glass transition temperature, it is difficult to satisfy both of the strength and flexibility of the coating film, and in turn, the press formability and the corrosion resistance after the shape processing thereof are insufficient.
In both of the techniques to be used for a coating film disclosed in Patent Document 3, where two or more different kinds of resins are mixed, and the technique for a coating film disclosed in Patent Document 4, where two kinds of resins are formed into two layers of an upper coating layer and a lower coating layer, the glass transition temperature is to be designed only for one layer, which is insufficient for the design of the glass transition temperatures for the two layers, and therefore, for the same reason as stated above, the press formability and the corrosion resistance after the shape processing thereof are insufficient. Also, the urethane resin having a low glass transition temperature disclosed in Patent Document 3 is in a rubbery state at an ambient temperature, and this incurs a problem that handling scratches are readily made.
For the purpose of improving press formability by imparting good flexibility and strength to the coating film, Patent Document 5 discloses a technique for a coating film containing two or more different kinds of organic solvent-based thermosetting resins having a glass transition temperature of 50° C. or less, and a glass transition temperature exceeding 50° C., respectively.
However, the technique of Patent Document 5 uses an organic solvent-based thermosetting resin and therefore, it has a problem that the environmental load is high and moreover, a high thermal energy is unprofitably required at the baking and curing steps therefor. Further, a resin having a glass transition temperature of 50° C. or less is contained in the coating film, which causes a problem that, for example, the scratch resistance or press formability varies according to a change in the use environment such as summer season and winter season, and a resin having a high glass transition temperature is not contained in the coating film, which causes a problem that, for example, when press forming as severe as involving a steel sheet temperature exceeding 100° C. is performed, the press formability or the corrosion resistance after the shape processing thereof is reduced.